If it was going flat, that means there was a leak somewhere. At the stem, somewhere on the rim strip, somewhere along the bead. It's really that simple. Nine times out of ten, it's the stem/core.
If you mounted the tire dry (without any sort of mounting fluid, like bubble juice, soapy water, Schwalbe Easy Fit, etc.) then there's a very good chance part of the bead didn't seat. I cannot speak to the efficacy of Bontrager rim strips, as tubeless tape has proven itself to be effective for tens of thousands of miles, and I see no need to try anything else.
Mounting tubeless is like anything else, it takes some practice to get it right. Few people can walk up to a derailleur and adjust it perfectly the first time they lay hands on it. I've had a few hours of cursing and whipping tire levers, because some tire won't seat, or a bead popped and blasted sealant everywhere. It happens. But I also swapped tires last week, including removing the current set, cleaning them, and folding them for storage, in less than 30 minutes. The set of Pirellis on the bike now lose about 1psi every 24 hours.
I mention this because IME, tubeless tires don't ride particularly well with tubes put in them. I only run a tube in a tubeless tire under dire circumstances-- if a tube is going in, it's to hold a boot in place, which means I'm only counting on the tube to get me as far as the LBS or home, whichever is closer.
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